Study of tau lepton decays involving K0S at Belle

Abstract

The τ lepton is the only known lepton massive enough to decay into hadrons via a weak vector current, and this provides a sensitive probe of the standard model parameters,

Vus

, αs and ms. However, because of the limited numbers of τ+τ− events, the hadronic decays involving kaons in τ lepton which is directly related to

determination has not been well studied for the decades. The Belle experiment provides the best environments for studying τ leptons because of the high luminosity of e+e− collisions result in the production of large numbers of τ+τ− events. We report measurements of branching fractions for inclusive/exclusive τ decays involving K0S → π−π+ with 669 fb−1 accumulated data at Belle detector at the asymmetric energy KEKB e+e− collider. The branching fraction measured for the inclusive decay is B(τ− → K0S X−ν ) = (0.910±0.001±0.046)%, where X− can be anything

for exclusive decays we report: B(τ− → π−K0S ν ) =(4.10 ± 0.01 ± 0.21) × 10−3 B(τ− → K−K0S ν ) =(7.09 ± 0.06 ± 0.41) × 10−4 B(τ− → π−K0S π0ν ) =(1.95 ± 0.02 ± 0.11) × 10−3 B(τ− → K−K0S π0ν ) =(7.31 ± 0.12 ± 0.49) × 10−4 We also studied decays involving two K0S: B(τ− → π−K0SK0Sν ) =(2.41 ± 0.04 ± 0.26) × 10−4 B(τ− → π−K0SK0Sπ0ν ) =(2.06 ± 0.23 ± 0.24) × 10−5 where B(τ− → π−K0SK0Sπ0ν ) is the first measurement. We also report the first observation of the channel τ− → f1(1285)-ν in π−K0SK0Sπ0 decay with its total branching fraction of (1.07±0.23±0.16)×10−5. With these results, we expect the most precise value of

can be achieved.